Sir Andrew is arguably one of the greatest mathematical biologists of the 20th Century. From his experimental work with Hodgkin, he developed a set of differential equations that provided a mathematical explanation for nerve impulses -- the "action potential". This work provided the foundation for the all of the current work on voltage-sensitive membrane channels, which are responsible for the functioning of animal nervous systems. Quite separately, he developed the mathematical equations for the operation of myosin "cross-bridges" that generate the sliding forces between actin and myosin filaments, which cause the contraction of skeletal muscles. These equations presented an entirely new paradigm for understanding muscle contraction, which has been extended to provide our understanding of almost all of the movements produced by cells above the level of bacteria.